The present invention relates generally to a fuel cell water management system, and more particularly to passively controlling the presence of water in a flow shifting fuel cell anode flowpath.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a proton-transmissive membrane is sandwiched between the anode and cathode to produce a layered structure commonly referred to as a membrane electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
One fuel cell configuration that is particularly useful is referred to as a flow shifting fuel cell system. In such a system, one or more stacks (or a divided stack) have their hydrogen (or other fuel) flowpaths configured to cycle the fluid containing the fuel back and forth through the stack such that ports that allow the flow of fuel to and from the stack can function as both fuel inlet and outlet, depending on the flow direction of the shifted fuel. As fuel flows back and forth through the stack in a semi-closed cyclical pattern, one port in the anode flowpath is accepting fuel into the stack, while another is passing the fluid out of the stack. Portions of the flowpath can be selectively closed off (i.e., dead-ended or dead-headed) to prevent the escape of the fuel during the back-and-forth cycling. A combination of valves or related flow manipulation devices can be used to effect the shift in flow direction, causing the role of the ports to reverse. After a certain number of cycles (such as, for example, when a nitrogen level has built up to the point where the hydrogen in the fluid is too diluted), a purging step can take place, as can the addition of fresh fuel.
During operation of a flow shifting fuel cell, water can build up in the anode flowpath. This may be due to (among other things) the diffusion of water from the fuel cell's cathode to the anode. If the amount of water present at the anode becomes too great, the anode can flood, causing a reduction in performance to drop. Excess water is further problematic in cold temperature situations, as prolonged exposure to such conditions may cause the water to freeze. Other forces can cause the anode flowpath to dry out. Thus, the design of a fuel cell requires that attention be paid to the amount of hydration to ensure that neither too much nor too little water be present.
Flow shifting fuel cell systems have advantages over other approaches, such as anode flowpath recirculation-based systems, for while both can be used to improve the hydration of anode flowpaths and the electrolytes, the recirculation-based system does so with recirculation pumps and other heavily-burdened components that, in addition to increasing system cost, weight and complexity, can wear out, thereby subjecting the system to greater maintenance concerns. In addition, the use of such pumps requires a source of power (for example, electrical power) that, being supplied by the operation of the fuel cells, reduces overall system efficiency.
Flow shifting fuel cells may use water separators as a way to manage the amount of water present in an anode or anode flowpath. Present water separation technologies are only configured to operate in one designated flow direction, which are incompatible with flow shifting configurations. Thus, to make present water separators compatible with flow shifting fuel cells, multiple separators with complex valving schemes are required. Such extra componentry is disadvantageous in that in addition to contributing to pressure drops (such as due to irreversible expansion) and timing issues within the flowpath, they introduce extra weight and cost. The use of numerous cooperating valves may also necessitate the use of actuators or controllers to determine when the valves need to be opened or closed.
It is desirable that a flow shifting fuel cell system provide the operability enhancements made possible through the use of bidirectional water separation techniques. It is further desirable that a water separator minimizes system complexity and maximize efficiency.